1. Technical Field
The present invention relates to patient positioning for radiotherapy, and more particularly, to a system and method for patient positioning for radiotherapy in the presence of respiratory motion.
2. Discussion of the Related Art
Radiation treatment or radiotherapy involves the treatment of a disease with radiation, especially by selective irradiation with x-rays or other ionizing radiation and by ingestion of radioisotopes. During radiation treatment, high-energy x-rays or electron beams are generated by a linear accelerator (LINAC) and directed towards a cancerous target anatomy. The goal of the treatment is to destroy the cancerous cells within the target anatomy without causing undue side effects that may result from harming surrounding healthy tissue and vital organs during treatment.
Radiation treatment typically takes place over the course of several sessions during which a delivered radiation dose is broken into several portal fields. For each field, a LINAC gantry is rotated to different angular positions, spreading out the dose delivered to healthy tissue. At the same time, the beam remains pointed towards the target anatomy, which had been placed in the isocenter of the beam by positioning the patient.
Patient positioning is typically achieved by placing a set of tattoos on the patient's skin and aligning the tattoos with a pair of lasers. This is accomplished by imaging the cancerous area of the patient with a computed tomography (CT) scanner and tattooing marks on the patient's skin corresponding to the location of the cancerous area in conjunction with the alignment lasers. The alignment lasers are included in a treatment room with the CT scanner, and the patient is aligned with the lasers while the patient is on a table so that the patient's tattoos line up with the lasers. In this way, a coordinate system of a CT treatment plan can be registered with the coordinate system of a LINAC for delivery of the radiation dose.
The skin marking system, however, has limited accuracy. For example, the skin markers are not rigidly connected to the target anatomy. Thus, a shift can easily occur between the external markers and internal target anatomy due to weight loss of the patient, thereby resulting in an imprecise delivery of the radiation dose. In order to overcome this, image guided radiation therapy (IGRT) has been introduced. IGRT employs medical imaging modalities such as x-ray, ultrasound, CT, or magnetic resonance imaging (MRI) in conjunction with a LINAC gantry for patient positioning.
When using IGRT with the above-mentioned modalities, the principle of patient positioning is mostly the same. For example, the acquired images show the current location of the internal target anatomy. The images are registered with the LINAC coordinate system, which is independent of the position of the patient table, thereby allowing the table and the patient to be moved into a position that aligns the target anatomy with the isocenter of the LINAC gantry.
Another modality for use with IGRT is portal imaging. In portal imaging, the LINAC directly acquires images with the same beam used for treatment. Portal imaging is based on combining film based portal imaging with computer image processing and registration techniques to yield patient positioning data. This modality has been particularly advantageous because one does not have to register a “set-up modality” with a “treatment modality”. Portal imaging does, however, have a few limitations in that soft tissue is more or less transparent to the LINAC's high-energy beam and portal images show only bone structures.
Although IGRT reduces the drawbacks associated with correlating external markers with internal targets, IGRT techniques consider the body as a rigid object during dose delivery. Thus, because the patient is breathing during treatment, the target anatomy may move several centimeters due to the patient's respiration. In order to take into account respiratory motion during radiation treatment, several suggestions have been made. One suggestion has been to a gate the treatment beam. In other words, to switch the treatment beam on or off with respect to the patient's respiratory motion. Another has been to track the target by moving the beam along the target using collimator leaves in a multileaf collimator, and yet another has been to adapt the treatment plan's dose distribution to the target's spatiotemporal distribution.
Some of these techniques have been proposed to be implemented with four-dimensional (4D) CT scanners. This, however, has proven to be undesirable as the cost of placing a 4D CT scanner in every treatment room is high and because pre-treatment 4D CT data may not necessarily give correct information regarding the patient's respiratory motion. In another technique, fluoro x-ray images have been taken at the same angle as that of a planned treatment field, thus enabling observation of target movement in the time sequence of two-dimensional (2D) projection images. However, for each field during a treatment session, the projected 2D target has to be re-determined requiring that the patient be moved between an x-ray treatment simulator and the LINAC.
In yet another technique, radiosurgery can be performed using a modified cyberknife. Radiosurgery with a cyberknife is a one-time procedure that uses a lightweight LINAC mounted to a robotic arm having six degrees of freedom. Two x-ray systems are mounted on the ceiling and floor of a treatment room and are used in conjunction with an optical tracking system and optical markers attached to a patient to track the position of the target during respiratory motion. The ongoing x-ray imaging of this technique is not appropriate for radiotherapy, as radiotherapy takes place over many treatment sessions and because it uses excess radiation. Moreover, the inclusion of two x-ray systems in a treatment room is a costly proposition.
Accordingly, there is a need for a technique of locating and tracking cancerous target anatomies in the presence of respiratory motion that reduces radiation exposure to nearby healthy tissue in a cost-effective manner.